The present invention relates to an apparatus for tracking an optical disk by means of sampling marks regularly distributed around the track and, more particularly, to an apparatus for tracking an optical disk capable of detecting a positive feedback region for a tracking error signal and thereby achieving steady tracking control.
As a method for tracking an optical disk, one in which reproduced signal are obtained from pits previously formed and distributed around the track at regular intervals wherein a tracking signal is obtained based on the reproduced signals from the pits is known. That is, as described in U.S. Pat. No. 4,562,564, pairs of pits are previously formed along an imaginary center line of the track in a wobbled manner and the tracking error signal is obtained based on the fact that the signal amplitude values obtained from the wobbled pits change according to relative distances between the light beam and the wobbled pits. A similar tracking method is also discussed in a paper published in SPIE, Proceeding, Vol. 695, Optical Mass Data Storage II (1986), pp. 112-115. This method uses sampling marks consisting of three pits as shown in FIG. 1. Referring to the figure, reference numerals 1 and 2 denotes a pair of pits for detecting a tracking error signal, which are offset from the imaginary center 4 of the track to opposite sides by a distance of 1/4 of the track pitch. Reference numeral 3 denotes a pit located on the imaginary center of the track for reproducing a clock signal.
The tracking error signal which is generated when a light beam moves across the tracks on an optical disk in its radial direction is made up of the difference between amplitude values of the reproduced signals from the pits 1 and 2. That is, a sinusoidal tracking error signal as shown in FIG. 2 is obtained, wherein points a, b, c represent the center of the track and the points e, f, g represent midpoints of adjoining tracks. At the point a, b, and c, the amplitude values of the signals from the pits 1 and 2 are equal to each other, whereby the tracking error signal becomes zero. Also, at the point e, f, and g, the amplitude values of the signals from the pits 1 and 2 on adjoining tracks become equal to each other, and therefore, an apparently similar tracking error signal to that generated in the center of a track is obtained, but the polarity of the signal is opposite. Hence, it results in a positive feedback for the servo system, and therefore, when the tracking servo system is closed while the light spot is located between tracks, it may occur that the servo oscillates and becomes unable to achieve alignment of the light spot on the track. Thus, a long time may be required for acquireing a steady tracking condition at the time of access.
In the above described prior art, any particular consideration was not given to the problem of such a positive feedback in the tracking servo system. As a result, there was a drawback that a long time was required before acquiring the steady tracking condition.
List of relative prior arts:
U.S. Pat. No. 4,562,564, U.S. Pat. No. 4,561,082,
U.S. Pat. No. 4,553,228, U.S. Pat. No. 4,432,083,
U.S. Pat. No. 4,489,406, U.S. Pat. No. 4,435,797,
U.S. Pat. No. 4,402,061, U.S. Pat. No. 4,443,870.